1. Field of the Invention
The present invention relates to a method of manufacturing a glass substrate suitable for incorporation in a magnetic recording medium such as those installed in hard disk drives for computers and consumer appliances. In particular, the invention relates to a method of manufacturing a glass substrate which imparts excellent durability and usefulness for forming a magnetic recording medium. The present invention also relates to a glass substrate manufactured by such a method. The present invention further relates to a magnetic recording medium using the glass substrate.
2. Description of the Related Art
Recently, rapid progress has been made in magnetic recording devices toward enhanced recording density and reduced costs, and random access operation carried out with a magnetic recording medium rotating at a high speed and running with a head having a minute flying height. In order to achieve both high density recording and high speed access in a magnetic recording device, it is necessary to reduce the distance between the magnetic disk and the recording head, that is, the head flying height, and, at the same time, to increase the rotation speed of the magnetic disk.
Magnetic recording media typically use primarily a substrate of aluminum having Ni—P plating thereon. In magnetic recording devices for mobile application, however, glass substrates are known to be advantageous for achieving high shock resistance, as well as the above requirements, since a glass substrate exhibits high rigidity and hardly deforms even at high speed rotation, and the surface thereof has a high smoothness.
With increasing demand for magnetic disk drives for use in household information appliances, the need is arising for insuring a plentiful supply of glass substrates and for further reducing the cost. Glass substrates, which can be easily formed to a disk shape by pressure molding at a temperature higher than the softening temperature, can be mass-produced at a low cost in that temperature range. In manufacturing the glass substrates, a lower molding temperature is advantageous for easy and simple pressure molding. Efforts to lower the molding temperature have been made by adding alkali metal(s) such as lithium, sodium, and potassium into the glass material.
Addition of alkali metal(s) into a glass substrate incorporated in a magnetic recording medium, however, causes alkali corrosion of the magnetic layer in the magnetic recording medium due to elution of the alkali metal from the glass substrate. The elution of alkali metal from the glass substrate occurs on the surface of the glass substrate in the form of precipitation of a carbonate compound formed by the alkali metal and carbon dioxide in the air. This situation may give rise to head crash by contact of the precipitates of alkali carbonate generated on the medium surface with the magnetic head. Therefore, the elution of alkali metals on the glass substrate needs to be suppressed as much as possible. In order to deal with this problem, the following means have been disclosed.
Japanese Unexamined Patent Application Publication No. H10-226539 discloses a technology in which, after lifting the glass substrate from a chemical tempering solution, the glass substrate in a heated state is washed with warm water (solvent) and salt crystals on the substrate are removed utilizing the polarity and thermal energy of the solvent.
Japanese Unexamined Patent Application Publication No. 2000-082211, which corresponds to U.S. Pat. No. 6,312,841, discloses a technology in which a substrate consisting essentially of glass for a magnetic recording medium is subjected to treatment for retarding elution of components of the glass by bringing the substrate in contact with a molten salt that contains hydrogen sulfate and/or pyrosulfate, while being subjected to a treatment for retarding crystallization of the molten salt.
Japanese Unexamined Patent Application Publication No. H10-194789, which corresponds to U.S. Pat. No. 6,134,918, discloses a technology in which the surface of a glass substrate lifted from a chemical treatment liquid is treated with a heated, water-soluble organic solvent, for example, glycerol or polyethylene glycol.
Japanese Unexamined Patent Application Publication No. 2004-059391 discloses a technology in which a glass substrate for a magnetic recording medium, containing alkali metals, is immersed in a melt of organic acid at a temperature ranging from 120° C. to 350° C. and then washed to remove the melt of organic acid or its solidified substance remaining on the surface of the glass substrate so that it is suitable for use in a magnetic recording medium.
Japanese Unexamined Patent Application Publication No. 2002-220259 discloses a technology in which a glass substrate is immersed in an aqueous solution containing lithium salt for exchanging the Li+ for Na+ and K+ on the glass substrate, and binding the Li+, which has a smaller ion radius than that of the Na+ and K+, with non-crosslinking oxygen, firmly in the glass to effectively suppress elution of the alkali metals.
Japanese Unexamined Patent Application Publication No. 2003-030828 discloses a manufacturing method for a glass substrate comprising a step for suppressing elution of alkali metals. In that method, after a chemical reinforcement treatment, the glass substrate is immersed in water under the conditions that the temperature is not lower than 180° C. and no higher than 30° C. above the temperature of the chemical reinforcement treatment, and that the water is pressurized to maintain a liquid state.
The technologies disclosed in the prior art documents discussed above, however, raise the following issues.
In the technologies disclosed in Japanese Unexamined Patent Application Publication No. H10-226539 and Japanese Unexamined Patent Application Publication No. 2000-082211, which corresponds to U.S. Pat. No. 6,312,841, hydronium ions (H3O+) generated from water included in the treatment liquid are exchanged for ions of the alkali metals in the glass substrate, thereby eliminating H2O and eventually substituting the alkali metals in the glass for hydrogen atoms. As a consequence, it could be assumed that the surface of the glass substrate is reformed and the alkali metals hardly precipitate on the surface even in an environment of high temperature and high humidity. Nevertheless, the technology of Japanese Unexamined Patent Application Publication No. H10-226539 reduces the alkali metal ions on the glass surface on the one hand, but glass skeletons on the glass surface are possibly destroyed by the high concentration of acid, increasing mobility of the alkali metal ions on the other hand. Therefore, elution of alkali metals may in fact be promoted. The technology of Japanese Unexamined Patent Application Publication No. 2000-082211, which corresponds to U.S. Pat. No. 6,312,841, treats the substrate in a molten salt at about 300° C., which may cause the substrate surface to become coarse.
In the technologies of Japanese Unexamined Patent Application Publication No. H10-194789, which corresponds to U.S. Pat. No. 6,134,918, and Japanese Unexamined Patent Application Publication No. 2004-059391, water is not used as a solvent so that no hydronium ions are generated and so that the glass substrate surface is not reformed. Consequently, there is a possibility that elution of the alkali metals is not prevented sufficiently.
In the technology of Japanese Unexamined Patent Application Publication No. 2002-220259, the solubility of lithium nitrate is 84.5 g in 100 g of water at room temperature, and the boiling point of the aqueous solution of lithium nitrate at this concentration is 113° C. When an aqueous solution at a concentration that allows a treatment temperature higher than 113° C. is used, the lithium nitrate solidifies at room temperature, causing deterioration of maintainability. A treatment at a temperature not higher than 113° C. may degrade ion exchange efficiency.
The technology of Japanese Unexamined Patent Application Publication No. 2003-030828 needs a high pressure vessel so that cost reduction of the fabrication apparatus is inhibited.
Although various technologies have been disclosed for substrates for magnetic recording media as described in the foregoing, demand still exists for a technology to obtain a glass substrate that exhibits excellent performance including durability by suppressing elution of alkali metals from the substrate.